How to sharpen the lawnmower blades with a file, step by step


Lawnmowers help to keep your lawn in good order. But after extended use the blades will become dull. You can notice this by some uncut patches or uneven cut grass. The fix for this problem is easy, sharpen the blade. You can do this at home with some simple tools like a cutting file. Files are a simple and readily available tool. Sharpening the blades with them requires some time and effort but is well worth it. The following blog will explain the steps in more detail.

How to sharpen lawnmower blades with a file, step by step:

  • Step 1. Gather the necessary tools.
  • Step 2. Disengage the plug and inspect the existing set of blades
  • Step 3. Remove the center nut of the blade carefully and clamp it
  • Step 4. Sharpen the blade with a file
  • Step 5. Check for the proper filing angle
  • Step 6. Balance the blade and fix it back

For all types of mowers, the blades can be found at the bottom of the machine. Most powered mowers have a single set of double-edged blades that can get dull after extended usage. The wear and tear will be even more when they contact stones, dirt, and water. A lawnmower loses its efficiency if its blades are not in good condition. Sharpening them will help prolong their life. Using file is a simple way to sharpening the blades manually. If you feel like taking up the job yourself without the help of an expert, the following blog post includes all the necessary steps and precautionary measures to help you do so.

General Step-by-step guide:

● Step 1. Gather the necessary tools

Using the right tools will help you finish the sharpening much quicker. To do the job, you need the following tools:

  • A pair of safety goggles and gloves.
  • Wrench set
  • A flat file
  • A round file
  • Clamps
  • Piece of wood
  • A bench vise to clamp the blades
  • Sandpaper
  • A sharpening or polishing stone

● Step 2. Inspection of existing lawnmower blades

Before starting with removing and inspecting the blade, you should disconnect the spark plug cables. This will ensure that the engine can not start unintentionally. You do not want to work on the blades without this safety measure. If you have an electric lawnmower, remove the batteries or disconnect the mains cable. Apply the parking brakes so that lawnmower stays stationary.

Lawnmower blades usually will come dull after around 30-40 hours of usage, depending upon the location where they are used. When you hit solid objects like rocks or metal, this can be even sooner. When they ready, this point sharpening will be necessary. Before sharpening the lawnmower blades condition should be checked. So you will understand the extent of damage and then decide whether sharpening is enough or blades need to be replaced.

The lawnmower blades are at the bottom of the lawnmower. That means that in order to reach them, the mower needs to be tilted. For heavy lawnmowers, this will be more difficult, and a lawnmower table can help. To prevent any fuel leak, close the fuel valve (if available), or empty the gas tank.

When you have access to the blade, check for rough edges of the blade. Try rotating the blade to and see if there is any hindrance offered to the rotation. Make sure to have your safety gloves on while inspecting. Check for severe cracks or the presence of rust. Mark your blade on one side, so you know which way to reinstall it.

● Step 3. Remove the center nut of the lawnmower blade carefully and clamp the blade

After you have turned the mower on its side (or upside down), loosen the nut at the center of the blade with a socket wrench. If it is difficult to loosen, spray the nut with some cleaner spray like WD-40 and wait few minutes. If the blade rotates while removing the nut, clamp a piece of wood to the lawnmower’s underside with C-clamps. This will keep the blade in place. After removing the nut, take off the blade from the lawnmower body.

Place the blade in a bench vise and tighten up as much as possible to hold it firmly in place. The blade comprises two cutting edges on opposite sides of one long piece of metal. You will have to work on each turn by turn.

● Step 4. Sharpen the lawnmower blade with a file

Before using a file, remove all grass, dirt, and debris from the blade with a stiff wire brush. If it is difficult to all the dirt and debris with the wire brush, rub the blades with sandpaper

Sharpening your blade with a hand file is not tricky. Tools like a flat-file cut in the forward stroke only, so you have to give regular thrusts keeping the correct angle with the blade surface. Mower blades are made from relatively softer steel. You can refine the edges, nicks mostly with fewer than 50 strokes of a clean, sharp mill bastard file approximately 10 inches long.

By gentle strokes, chip off the rough blade surface going inside out in regular motion. Maintain the geometry as you go from the corner to the center, keeping the blade profile constant. Repeat the process for the other side of the blade too. The blade gets sharpened when the dull black surface gets chipped off to reveal a shinier and smoother surface.

Note: Angle and bench grinders are also used and work much more quickly (professionals use them). However, they’re harder to control, and you might overheat and damage the blade. So, a file is preferred due to the ease of operation and lesser professional experience required.

● Step 5. Check for the Accurate Filing Angle

Always use the file at the correct angle according to what the manufacturer’s described in the lawnmower user manual. Remember to always use the file along the natural angle. Otherwise, you can remove too much metal. Also, rub the file from the center of the blade towards the tip. In this way, you get a sharp tip and avoid any chances of creating cracks.

In the end, give a few pushes using a round file at the blade edge periphery. This will round the sharp tips to some extent. This particular step is crucial to prolong the blade’s life. If the tips are too sharp, they will get damaged too soon. You can use the round file along the blade length shaping the cutting edge. In the end, you can simply use a clean piece of cloth with some cleaner spray to polish the blades.

● Step 6. Balance the blade and fix it back

For the machine and user’s safety, the rotating blade must be perfectly balanced. You can check the balance by hanging the blade on the nail on the wall horizontally. Let the blade hang on the support and notice that both sides should be hanging equally. File the heavier end if it is lower. This step prevents any vibration and noise from the machine.

Fix the blade back by applying some grease to the central nut too. Check if the blades are rotating freely with no obstructions. Reconnect the spark plug and start the mower. Let it run for a while and check if you notice any strange sounds. If such is the case, take it to an expert. Otherwise, you are ready to go.

Frequently asked questions

1. How are the lawnmower blades threaded?

Most lawn mowers blades are reversely threaded, which helps them secure on the engine’s driveshaft. The purpose of this is to make sure that the blade’s rotation while the cutting action does not loosen. Instead, it keeps the connection tightened by the nut.

2. Can a mower blade be too sharp?

Mower blades should be aggressively sharp but not as sharp as a razor’s edge. You should be able to touch the blade with your hand without getting cut. Additionally, lawnmower blades that are too sharp get duller, faster, resulting in sharpening more frequently and shorter blade life.

3. Is it dangerous to sharpen the lawnmower’s blade by hand?

No, one can easily clean and sharpen the lawnmower’s blade by hand. All you need is to wear proper safety gloves and carefully remove the blade from the lawnmower. Clean the blades with a sandpaper or wire brush. More caution is to be taken when the blades are to be reinstalled, as they will be sharper at that time compared to removing.

Final remarks

An evenly cut grass is every gardener’s obsession. A dull lawnmower blade proves an obstruction to achieving this. Sharpening the blades will surely remove problems like left-out patches of irregular grass. You can easily sharpen your lawnmower blades with the use of a flat-file. The file chips off the rough surface, excavating the sharper metal. By following the user-friendly guide from above, you can easily regulate your mowing experience through proper precautions.

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